Matthew Hutchison dresses as the Obamacare website as he participates in the West Hollywood Halloween Costume Carnaval, which attracts nearly 500,000 people annually in West Hollywood, California October 31, 2013.

The Congressional Budget Office, which originally estimated that the Affordable Care Act, or "Obamacare," would reduce the national debt by $120 billion, now says it has no way of estimating the cost of the new health care law.

"CBO and [Joint Committee on Taxation] can no longer determine exactly how the provisions of the ACA that are not related to the expansion of health insurance coverage have affected their projections of direct spending and revenues. The provisions that expanded coverage established entirely new programs or components of programs that can be isolated and reassessed. Isolating the incremental effects of those provisions on previously existing programs and revenues four years after enactment of the ACP is not possible," the footnote read.

When President Barack Obama was trying to get the law passed in 2010, he frequently cited a CBO report estimating that the law would reduce the deficit by $210 billion over 10 years.

Republicans argued at the time that the estimate was wrong because the CBO was double counting cuts to Medicare as both reducing the deficit and funding other programs in the law. (An independent actuary hired by the Department of Health and Human Services in 2011 confirmed that money used to fund other programs in the law cannot simultaneously be used to lower deficits.)

The costs of the ACA are hard to estimate, in part, because President Barack Obama keeps making changes to the law, 41 changes so far, according to the Galen Institute. Many of these changes change the law's financial impact.

In 2011, for instance, HHS decided not to implement CLASS, a nursing home and home care program, because it was found to be unfeasible. Over half the ACA's estimated savings, $86 billion worth, was supposed to come from this one program.

More recently, Obama has delayed and made changes to the individual mandate and employer mandate, both of which were intended to lower the costs of the ACA. Additionally, implementation of the new law has not gone smoothly. Difficulties with the new website, healthcare.gov, will add costs that were unanticipated.

Joseph Antos, a former CBO health care expert who now works for the American Enterprise Institute, a conservative think tank, suggested that the CBO does not want to come up with a new estimate because doing so would make its 2010 estimate look bad.

"The reality is that they were able to do it a year and a half ago or two years ago, so you have to wonder what changed? They don't want to admit that what they assumed two years ago is no longer correct because the administration has not implemented many provisions of the law," he told Roll Call.

Bill Hoagland, a budget expert and senior vice president for the Bipartisan Policy Center, told The Fiscal Times that the CBO footnote was not "terribly exciting" from an analyst's perspective, "but politically, I think it's dynamite."